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Subject: Re: [PATCH] task_work: only grab task signal lock when needed
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jann Horn <jannh@google.com>
References: <0028d3ea-4d05-405f-b457-75c83d381d89@kernel.dk>
 <20200811152310.GF21797@redhat.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <6b43293f-2052-2bd8-80ef-eb936fe19b65@kernel.dk>
Date:   Tue, 11 Aug 2020 10:45:21 -0600
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On 8/11/20 9:23 AM, Oleg Nesterov wrote:
> On 08/11, Jens Axboe wrote:
>>
>> --- a/kernel/task_work.c
>> +++ b/kernel/task_work.c
>> @@ -42,7 +42,8 @@ task_work_add(struct task_struct *task, struct callback_head *work, int notify)
>>  		set_notify_resume(task);
>>  		break;
>>  	case TWA_SIGNAL:
>> -		if (lock_task_sighand(task, &flags)) {
>> +		if (!(READ_ONCE(task->jobctl) & JOBCTL_TASK_WORK) &&
>> +		    lock_task_sighand(task, &flags)) {
> 
> Aaaaah, sorry Jens, now I think this is racy. So I am glad I didn't add
> this optimization into the initial version ;)
> 
> It is possible that JOBCTL_TASK_WORK is set but ->task_works == NULL. Say,
> task_work_add(TWA_SIGNAL) + task_work_cancel(), or the target task can call
> task_work_run() before it enters get_signal().
> 
> And in this case another task_work_add(tsk, TWA_SIGNAL) can actually race
> with get_signal() which does
> 
> 	current->jobctl &= ~JOBCTL_TASK_WORK;
> 	if (unlikely(current->task_works)) {
> 		spin_unlock_irq(&sighand->siglock);
> 		task_work_run();
> 
> nothing guarantees that get_signal() sees ->task_works != NULL. Probably
> this is what Jann meant.
> 
> We can probably add a barrier into get_signal() but I didn't sleep today,
> I'll try to think tomorrow.

Appreciate you looking into this! Would be pretty critical for me to get
this working.

-- 
Jens Axboe